1. Field of the Invention
The present invention relates to a tight contact type document reading apparatus used for a facsimile machine, a card reader, or an image scanner used for image input to a computer.
2. Description of the Related Art
Generally, a document reading apparatus reads an image in a direction perpendicular to a moving direction as moving document relatively to the apparatus. In reading an image, conventionally a charge coupled device (CCD) with a plurality of photoelectric transducer elements arranged in one dimension was often used. The pitch of a photoelectric transducer is, for example, 7.mu.. The number of photoelectric transducer elements that can be installed on a CCD is approximately, 5000 at present. A pixel density required for reading a document is, for example, 16 pixels/nm. Under such a condition, an image from a document must be scaled down by an imaging optical system when it is received by a CCD.
U.S. Pat. No. 4,774,592 discloses a construction for reading an image by pixels more than the number of photoelectric transducer elements of a CCD. In FIG. 1, a plurality of convex lenses 2a, 2b are disposed at an interval with a base plate 1. CCDs 3a, 3b are mounted on the base plate 1 corresponding to the convex lenses 2a, 2b. Each convex lense 2a, 2b forms an image of a document 4 on each CCDs 3a, 3b including an overlapping portion of length .DELTA.L. The overlapping portion is provided in order to eliminate an influence by thermal expansion of the base plate 1. The temperature of the base plate 1 is detected by a thermistor 5. Output from CCDs 3a, 3b is processed to remove the overlapping portion corresponding to the detected temperature.
A document reading apparatus using CCD has a pitch of photoelectric transducer elements considerably smaller compared with a required pixel density, so that light path necessarily becomes long (shrinkage rate becomes high) and a large form of imaging optical system is required. Long light path needs more supporting members for each parts so that it becomes difficult to design and to manufacture products resulting in an increase in cost. Long light path (high shrinkage rate) also amplifies minute deviation from optically designed arrangement. It is therefore very difficulty to control variation of manufactured parts to adjust coefficient of thermal expansion, and to compensate light axis. When a plurality of CCDs are used, influence of thermal expansion must be removed as described above. In case a single CCD is used to read along document, the light path becomes longer and the shrinkage rate becomes higher. This makes it inevitable that and power of definition become low around periphery of the lens. Therefore, lately a document reading apparatus such as a tight contact type image sensor has been actively developed without using a CCD. A typical prior art of a tight contact type image sensor is disclosed, for example, in JAPANESE LAID-OPEN PATENT 62-18166 (1987).
Such a conventional document reading apparatus is normally composed of a light source 6 consisting of a fluorescent lamp or the like, a rod lens array 2 of an erect equi-magnification optical system, and a semiconductor chip 7 having a plurality of photoelectric transducer elements 7a, 7b as shown in FIG. 2. The light source 6 projects light upon a document 4, and the reflected light is radiated upon each photoelectric transducer element 7a of the semiconductor chip 7 through the rod lens array 2. An image of the document 4 is read by causing a photoelectric transformation corresponding to the light radiated upon each photoelectric transducer element 7a.
However, in this conventional document reading apparatus, the rod lens array 2 to guide the reflected light from the document 4 upon the photoelectric transducer elements 7a, 7b on the semiconductor chip 7 has an erect equi-magnification optical system. Since the reflected light from the document 4 is radiated upon the photoelectric transducer elements 7a, 7b at an equal magnification, the photoelectric transducer elements 7a, 7b on the semiconductor 7 must be arranged in a straight line at a specified interval substantially with the same length as the document (e.g., 216 mm in the case of size A4). Accordingly, such a document reading apparatus had a defect that the photoelectric transducer elements 7a, 7b must be arranged in a very long line.
Also the semiconductor chip 7 with a long size arrangement of the photoelectric transducer elements 7a, 7b at a specified interval is very hard to manufacture by the present semiconductor manufacturing technology. Generally a nondefective semiconductor chip with a short length is cut out from a semiconductor base plate with a certain size, and a plurality of such semiconductor chips 7 with a short length are arranged in a straight line. Thereby the photoelectric transducer elements 7a, 7b are arranged substantially with the same length as that of the document 4.
However, when the nondefective semiconductor chip 7 is cut out from a semiconductor plate, the photoelectric transducer elements 7a, 7b are formed over the whole range, so that a stress during cutting out is applied to the photoelectric transducer elements 7a, 7b in the vicinity of cutting portion, thereby causing a distortion. As a result, such a document reading apparatus has a defect that precision for reading a document decreases because precise photoelectric transformation could not be made by the photoelectric transducer elements 7a, 7b.
Here, in order to solve such a defect, it may be possible to reduce the area of each photoelectric transducer element 7a, 7b formed on the semiconductor chip 7, keeping a long distance between the cutting portion of the semiconductor chip and the photoelectric transducer elements 7a, 7b in the vicinity thereof. However, when the area of each photoelectric transducer element 7a, 7b is reduced, the amount of transformation electricity at each photoelectric transduction element 7a, 7b becomes small, which leads to a defect of falling sensitivity in document reading.